Automatic sprinkling systems are well known in the art. These systems typically operate in a manner where the user specifies a certain time for starting a watering cycle as well as a length of time that each specified watering cycle (in perhaps each of a number of watering zones) should last. In most situations, the user makes an initial best guess as to the watering needs of the vegetation, and sets a watering time based on that guess. Thereafter, the user monitors the health of the plants and condition of the soil in each watering zone to make adjustments to the initial guess as to watering needs and causing corresponding adjustments to the specified watering time for each zone. Ideally, an automatic sprinkler system should return to the vegetation only as much water as has been lost through either evaporation from the soil or transpiration from the plant. This manual guessing process for specifying sprinkling durations is notoriously inaccurate. In fact, some estimates indicate that automatic sprinkler users over-water their vegetation by as much as a thirty to forty percent (30-40%) factor.
The term “evapotranspiration” (ET) refers to the amount of water a plant uses or needs in order to maintain growth. The climatic information commonly used to calculate an evapotranspiration value include temperature, solar radiation, wind speed, and vapor pressure or humidity. This climatic information is generally collected by a full service weather station and processed in one of a number of known complex formulas or equations to calculate the evapotranspiration value. One example of such an equation recognized in the agriculture industry for accuracy in measuring evapotranspiration using weather station collected climatic information is the well-known Penman-Monteith or modified Penman equations (hereafter referred to as “modified Penmans”).
Considerable effort has accordingly been expended in developing evapotranspiration formulas which mimic the results provided by the weather station climatic information driven (e.g., Penman-Monteith, modified Penmans) equations, but do not require access to such large amounts of specific weather station collected climatic information. More specifically, there is a need for an accurate evapotranspiration formula which requires for its input data climatic information that is easily and inexpensively collectable at the specific site where the vegetation at issue is located. One such formulation comprises the Hargreaves equation as set forth below:
wherein:ET0=0.00009×RA×(T° C.+17.8)×TD0.50 ET0=reference evapotranspiration (in inches of water per day); andRA=extraterrestrial radiation expressed in equivalent evaporation (in inches of water per day);and further wherein:
                              T          ⁢                                          ⁢          °          ⁢                                          ⁢                      C            .                          =                  average          ⁢                                          ⁢          daily          ⁢                                          ⁢          temperature                                                  =                                    (                                                T                  ⁢                                                                          ⁢                  max                                +                                  T                  ⁢                                                                          ⁢                  min                                            )                        /            2                          ;        and                                TD        =                  daily          ⁢                                          ⁢          temperature          ⁢                                          ⁢          differential                                        =                              T            ⁢                                                  ⁢            max                    -                      T            ⁢                                                  ⁢            min                              
Hargreaves techniques have been improved upon by some conventional systems that collect daily high and low temperature data at the site of an irrigation controller. This temperature data is then processed, along with extraterrestrial radiation influenced equivalent evaporation data, in accordance with the Hargreaves equation, to determine a reference evapotranspiration value which represents an estimation of the current watering needs of a certain reference vegetation at the site. The reference evapotranspiration value is then adjusted by a local deviation factor specific to the site which affects evapotranspiration rates to generate an adjusted evapotranspiration value. More particularly, this local deviation factor accounts for any deviation between actual evapotranspiration or weather station climatic information driven evapotranspiration and the Hargreaves equation calculated evapotranspiration, and thus adjusts for localized errors in the application of the Hargreaves equation. The locally adjusted evapotranspiration value is then further adjusted, for example, to account for the type of vegetation at the site to generate a net evapotranspiration value representing an estimation of the current watering needs for the specific plants at that specific site. The net evapotranspiration value is then divided by a sprinkler head average precipitation rate to determine a run time for irrigation. The controller then irrigates the site for the duration of the determined run time necessary to satisfy the watering needs of the vegetation. Of course, the Hargreaves equation is only one technique that can be used to generate irrigation schedules.
Some conventional systems use a distributed architecture to control irrigation. One conventional example includes a system having a controller that actuates valves at one or more irrigation sites. The controller is in communication with a central server over, e.g., the Internet, where the central server calculates irrigation schedules using weather data and sends those irrigation schedules to the controller. The controller saves received irrigation schedules and opens and closes the irrigation valves according to the received irrigation schedules. However, such a solution may not be adequate for some users. For instance, if the communication link is broken between the controller and the central server, then the controller might not operate optimally. Also, such conventional system may rely on weather data generated off-site, which may be less reliable than weather data generated on-site.